Self and Cross pollination Atul Bhargava Department of Botany Mahatma Gandhi Central University Motihari-845401, Bihar INDIA Programme: B.Sc (H)- Botany Course: Plant Breeding (BOTY3054) Self pollination Self pollination Involves the transfer of pollen grains from the anthers to the stigma of the same flower or of another flower borne by the same plant. It is of two types : (i) Autogamy • Pollen from the anthers of a flower are transferred to the stigma of the same flower. (ii) Geitonogamy • Pollen from the anthers of one flower are transferred to the stigma of another flower borne on the same plant. • Involves two flowers belonging to the same parent plant. • Usually occurs with monoecious condition. Different forms of self pollination: Autogamy and Geitonogamy Merits • Pollen grains are not wasted. • The purity of the generation is maintained. Demerits • New and healthier varieties are not formed. • Results in weak progenies, producing weaker seeds and plants. Contrivances for self pollination Contrivances for self pollination (a)Bisexuality • Flowers should be bisexual or hermaphrodite. (b) Homogamy • Anthers and stigma of the bisexual flowers mature at the same time. • They are brought close to each other by growth, bending or folding to ensure self pollination. • This condition is called homogamy. Mirabilis (Four O, clock) Catharanthus Potato Sunflower Wheat (c) Cleistogamy • Some plants never open to ensure complete self-pollination. Examples: Commelina bengalensis Oxalis Viola • Cleistogamous flowers are: bisexual small Inconspicuous colourless do not secrete nectar Cross pollination Cross pollination (Xenogamy) Involves the transfer of pollen grains from the flower of one plant to the stigma of the flower of another plant. Merits Seeds are more and viable. Progenies are healthier. Adaptability is better. New varieties can be produced. Demerits The process is not definite because plants depend on external agencies. Large amount of pollen grains are wasted. Cross pollination (Image courtesy: Shutterstock.com) Contrivances for cross pollination Contrivances for cross pollination • Nature favours cross pollination. • All unisexual flowers and a large number of bisexual flowers are naturally cross pollinated. The main contrivances ensuring cross pollination are as follows: (i) Diclincy or Unisexuality: In unisexual flowers stamens and carpels are found in different flowers. Unisexuality can be of two types: Monoecious plant : When male and female flowers are borne on the same plant. e.g., Maize, Cucurbits, Castor. Dioecious plant : When male and female flowers are borne on different plants. e.g., Carica papaya, Cannabis. (ii) Dichogamy • In bisexual flowers, when two sexes mature at different intervals and thus avoid self pollination is known as dichogamy. Protandry • When stamens mature earlier than the stigma. Examples: Coriander Jasmine Sunflower Lady’s finger Protogyny • When stigma matures earlier than the stamens. Examples: Rose Tobacco Crucifers (iii) Heterostyly • Flowers are dimorphic. • Pin-eyed: long style but short stamens. • Thrum-eyed: short style and long stamens. • Example: Oxalis. Heterostyly (iv) Herkogamy • Stigma and anthers mature at the same time. • Self pollination is avoided by some sort of barrier. • The flowers show following contrivances : The male and female sex organs lie at some distance from each other. In some flowers corolla has peculiar forms which act as barrier in self pollination. e.g., Aristolochia. In some other flowers, the pollens are held together to form pollinia which can only be carried away by insects. Example: Orchids and Calotropis. (v) Self sterility or Incompatibility • Pollen grain of an anther do not germinate on the stigma of the same flower. • Such flower is called self sterile or incompatible and this condition of flower is called self sterility. • Also termed as intraspecific incompatibility or self incompatibility. • In these flowers cross pollination is the only means for fertilization and production of seeds. Agents for cross pollination Agents for cross pollination There are two main groups of agents: (i) Abiotic Wind Water (i) Biotic Insects Birds Bats Snails Abiotic agents (a)Anemophily • Pollinated by wind. • Flowers are: small and inconspicuous with long and versatile stamens. • Examples: Sugarcane Maize Wheat Bamboo Pinus Papaya Grasses Mulberry Chenopodium (b) Hydrophily • Pollination takes place through water. • All aquatic plants are not hydrophilous. Hydrophily is of two types: Hypohydrophily Plants which are pollinated inside the water. Examples: Zostera Ceratophyllum Najas Epihydrophily Plants which are pollinated outside the water. Examples: Vallisneria (Ribbon weed) Biotic agents (a)Entomophily • Pollination brought about by insects. • Moths, beetles, butterflies, wasp, etc. • Flowers: brightly coloured, sweet smell, nectar. • Flowers produce a small amount of pollen which has a spinous and sticky exine. • The stigmas of such flowers are long rough and sticky. Examples: • Salvia: Lever or turn pipe mechanism • Yucca (by Tageticula moth), • Orchid Ophrys speculum (by Colpa aurea, a hairy wasp), • Ficus (by Blastophega) (b) Ornithophily • Pollinated by birds. • Common bird pollinators: Sun bird, Humming bird, Crow, Bulbul, Parrot, Mynah, etc. • Examples: Bombax (red silk cotton), Erythrina (Coral tree), Callistemon (Bottle brush), Bignonia, Agave, etc. • Flowers: brightly coloured; plenty of nectar; large quantities of pollen. • Humming bird pollinates while hovering over the flowers and sucking nectar. • The bird can derive about half of its body weight of nectar in a single day. • The nectar is chiefly made of sugars and provides a sweet drink to the bird. Bombax (red silk cotton) (Image courtesy: Shutterstock.com) Bignonia (Image courtesy: Wikipedia) (c) Chiropterophily • Pollination performed by bats. • Flowers: large, dull-coloured, have a strong scent. • Chiropterophilous flowers produce abundant pollen grains. • Flowers secrete more nectar than ornithophilous flowers and open at night emit a good fragrance. • Examples: Kigelia pinnata (Sausage tree) Adansonia (Baobab tree) Bauhinia megalandra Anthocephalus (Kadamb tree) Adansonia (Image courtesy: Wikipedia) Kigelia (Image courtesy: Wikipedia) (d) Malacophily • Pollination by slugs and snails. • Land plants like Chrysanthemum and water plant like Lemna shows malacophily. (e) Myrmecophily • Pollination by ants. • Example: Anemone nemarosa. .